Light-emitting diodes (LEDs), owing to their low power consumption and longevity compared with conventional luminescent devices such as light bulbs and fluorescent lamps, have seen a rapid growth in use in recent years. The fabrication of optical semiconductor devices includes the steps of applying, to specific positions on a substrate, a curable resin composition called a die attach material for attaching LED chips to the substrate, placing LED chips on top of the applied resin composition, and subsequently curing the resin composition. A wire bonding step is then carried out whereby electrode pads on the LED chip that are usually formed of gold are bonded to electrically conductive sites on the substrate with gold wire. The wire bonding step presses a gold ball that forms at a capillary tip against an electrode pad, while at the same time applying ultrasound. However, if the LED chip is not fully attached to the substrate, the ultrasonic waves disperse to the surroundings during pressing, as a result of which the gold wire cannot bond. A curable resin composition that provides a high-strength cured product could be used as the die attach material in order to fully attach the LED chips to the substrate.
By virtue of their heat resistance and light resistance, addition-curable silicone resin compositions such as methyl silicone systems are commonly used in die attach materials. However, it has been reported that, due to various factors such as the types of addition-curable silicone resin composition and LED chip used and the resin composition curing conditions, contaminants form on the gold electrode pads of the LED chip during cure of the resin composition. When contaminants are present on the electrode pads, this adversely affects the subsequent wire bonding step. Such contaminants are attributed to low-molecular-weight siloxanes present within the addition-curable silicone resin composition. In particular, with the application of heat during the cure, low-molecular-weight siloxanes having silicon atom-bonded hydrogen atoms (SiH groups) thereon vaporize, forming a film on the electrode pads due to hydrolysis reactions and the like, or they gel and deposit as contaminants on the pads. It is known to be possible, by lowering the content of SiH group-containing low-molecular-weight siloxane within the resin composition, to decrease the amount of contaminant that deposits on the LED chip electrode pads during heat-curing and thus improve wire bondability.
For example, JP-A 2008-255227 discloses an addition-curable silicone resin composition which, in addition to non-functional low-molecular-weight siloxanes (e.g., D3, D4), includes also a large amount of low-molecular-weight siloxane containing reactive SiH groups. This publication teaches in particular that, by setting the amount of low-molecular-weight siloxane compounds having a degree of polymerization of 10 or below and containing at least one SiH group on the molecule to not more than a given weight percent of the overall silicone resin composition, contaminant deposition to the surroundings during heat curing can be suppressed. Moreover, the examples described therein mention the removal, at reduced pressure and under nitrogen bubbling, of SiH group-containing low-molecular-weight siloxanes from the organohydrogenpolysiloxane serving as an ingredient of the addition-curable silicone resin composition, followed by the use of a thin-film distillation unit. However, this series of operations takes a great deal of time. Nor is any mention made of, for example, the resin yield following use of a thin-film distillation unit. Hence, for reasons of cost and convenience, further improvements have been desired.
JP-A 2010-174234 reports that addition-curable silicone resin compositions in which an MQ resin having alkenyl groups on the M units serves as an essential ingredient, because they contain no silicon atom-bonded aryl groups such as phenyl groups as substituents on the various constituent organopolysiloxanes, provide cured silicone products that are flexible and retain a good transparency even at elevated temperatures. However, when these addition-curable silicone resin compositions are used in applications requiring a high resin strength, such as die attach materials for optical semiconductor devices, the strength of the resulting cured silicone product leaves something to be desired.
Hence, when conventional addition-curable silicone resin compositions are used as die attach materials for optical semiconductor devices, contaminants deposit on the electrode pads of the LED chips, resulting in a decrease in wire-bondability.